Hi

Polyethylene is fine - those are the three major 
archival polymers ^_^. Since you'll use that, you 
should probably cut more "fitted" shapes and sizes
to back each layer since the larger sheets look 
*very* bad when they start to warp. -_-' I don't 
suppose you know what mil thickness the polyeth 
bags are - that particular polymer gets cloudy 
with increasing thickness, another reason to cut 
to size .. 

Later :P